This invention generally relates to the field of optical switching. More specifically, the present invention relates to optical systems comprising hollow optical waveguides and micro-mechanical deflectable mirrors.
Optical switching is a fundamental operation that is useful in optical communications, optical memory, and optical displays. In its most basic form, optical switching involves spatial redirection of information-carrying optical waves. FIG. 1 (prior art) shows a simple and generic optical switch 100. This one-inputxe2x80x94two-output optical switch directs input optical signal 101 to one or both of the two optical output ports, output 1102 and output 2103, under the command of a control signal 104.
This switch may be generalized to N-output ports. FIG. 2 (prior art) illustrates how a Ixc3x97N switch 200 may be constructed from multitude of 1xc3x972 optical switches 100. In this particular illustration, N equals 6. In the Ixc3x97N configuration, a switch may be used in a signal distribution network. Alternatively, by sequentially switching the optical signal to the N output ports that are arranged in a 2-D spatial array, the switch may be used in a display device where the input optical signal contains image information. The same switch may also be used to access different spatial locations in an optical memory.
A basic switch 300 may also be generalized to contain several input ports as illustrated in FIG. 3 (prior art). Again, a simple switch may contain two input ports, input 1301 and input 2302, as well as two output ports, output 1303 and output 2304. This may be a generalization of switch 100 shown in FIG. 1. Under the direction of an external control signal 305, each of the input signals, 301 and 302 may be directed to one or both of the output ports, 303 and 304 respectively. When a single input is distributed to both output ports, the operation is called xe2x80x9cbroadcast or fan-out.xe2x80x9d When both input signals are directed to a single output port, the operation is termed xe2x80x9cconcentration or fan-in.xe2x80x9d When each output port receives one or the other of the input signals, the operation is termed xe2x80x9cpermutationxe2x80x9d.
This switch may be generalized when the number of input signals and output ports exceed two. In general, the number of input signals, say M, and the number of output ports, say N, do not have to be equal. Such an Mxc3x97N optical switch forms the heart of an optical switching network in a communication system or the interconnection fabric inside a high performance computing system. Again, one skilled in the art can easily see how such an Mxc3x97N optical switch may be constructed from M optical,switches, each of which is a Ixc3x97N switch of the type shown in FIG. 2. Corresponding output lines from each of the M such switches may be combined to generate one of N output lines.
Performance parameters associated with such Mxc3x97N optical switches includes but is not limited to: the size of the switch (values of M and N); the bandwidth of the optical signals the switch can carry; the rate at which the signals can be redirected or the switch can be reconfigured; the optical efficiency of the switch (loss); the isolation between ports (cross talk); the ability to perform a broadcast operation; the ability to handle optical signals of different formats (analog, digital, AM, FM, PM, multiple wavelengths); the complexity of the control signal interface; the ease of interfacing to optical fibers; the overall size; the weight; the power consumption; the mechanical stability; and the ease of manufacturing. Each performance parameter is not equally important to different applications. In fact, some of them may not be relevant to certain applications at all. For example, the ease of interfacing to optical fibers may not be relevant to display applications. To the extent that some of these parameters may not be optimized simultaneously, the trade-off between them may be governed by the specific applications for which they are being designed.
Given the widespread applications of optical switches, it is not surprising that over the years a number of different designs have been proposed and implemented. These designs primarily fall in two categories: designs where light propagates in guided wave structures inside the switch and designs where light propagates in free space inside the switch.
Guided Wave Optical Switches
FIG. 4A and FIG. 4B (prior art) show schematic diagrams of a simple 1xc3x972 optical switch involving mechanical movements and using optical fibers. As illustrated, the type of switch may have an input fiber 402 and two output fibers 404 and 406, switched by mechanical movements 410 and 412. FIG. 4A shows input fiber 402 switched to output fiber 404 and FIG. 4B shows input fiber 402 switched to output fiber 406. This type of switch may be extremely simple to construct, have low loss, have very high isolation between ports, and carry full bandwidth available to optical fibers regardless of the format. However, this switch may be slow to reconfigure due the difficulty of moving a large mass, may not allow broadcast and may not be easily extendable to an Mxc3x97N switch. Still, such switches, due to their simplicity have found uses in fiber optic networks.
A second type of guided wave switches based on optical propagation in planar (integrated optic) waveguides and dynamic directional couplers is illustrated in FIG. 5 (prior art). This schematic diagram of a 1xc3x972 switch is based on a directional coupler. Light from a single mode waveguide 520 may be coupled into another single mode waveguide 522 that is brought in close proximity, allowing interaction between each waveguide, 520 and 522, via evanescent field coupling. By controlling the phase of the evanescent wave in the intervening region using a control mechanism 510, the coupling efficiency may be modulated between 0 and 100% (for ideal devices). If the inputs 502 and 506 to both waveguides contain optical signals, the same switch may be made to behave like a 2xc3x972-permutation switch, where the outputs are illustrated as 504 and 508. The phase of the evanescent wave may be controlled at the control mechanism 510 via electrooptic effect or thermooptic effect. Several such 2xc3x972 switches may be combined in a tree-like structure to realize an Mxc3x97N permutation switch. Such integrated optic crossbar switches have been commercially available from a number of companies including Ericsson of Stockholm, Sweden, Lucent of Murray Hill, N.J., and Worldwide Telecommunications Corporation (NTT) of Tokyo, Japan. These switches may be compact, switched extremely fast (if electrically controlled), have good isolation, and carry high bandwidth optical signals. However, these switches may also be difficult to couple to fibers, be environmentally sensitive, not achieve broadcast functionality, and not extend easily to large sizes without consuming a large integrated optic chip area.
FIG. 6 (prior art) illustrates an experimental approach to integrated optical switches utilizing electrooptically-activated Bragg reflectors embedded into waveguide junctions. Two one-dimensional arrays including a first input optical waveguide 610, a second input optical waveguide 612, a first output optical waveguide 614, and a second output optical waveguide 616 may be arranged orthogonally oriented to each other in a single plane as shown in FIG. 6. With no electric field activating a grating (620, 622, 624, or 626) at a junction, light injected in a waveguide may continue propagating along it. When the grating is activated, light may be scattered into the output waveguide that crosses the input waveguide. FIG. 6 shows this switch structure schematically. The figure shows a 2xc3x972 switch where input signal 1602 is coupled to output port B 608 and input signal 2604 is coupled to output port A 606. This is achieved by activating gratings 624 and 622, while leaving gratings 620 and 626 deactivated.
The advantages and disadvantages of this switch design are somewhat similar to the design shown in FIG. 4. Since this design involves waveguides at right angles, it may lead to a more compact switch. However, the cross talk characteristics may be distinctly inferior to the previous design and the build up of cross talk may limit the size of the switch.
Free Space Optical Switches
FIG. 7 (prior art) is a diagram of a free space optical switches. With free space optical switch designs, light beams propagate in free space without any guiding structures. Usually such systems employ lenses to keep the divergence of optical beams in check. An Mxc3x97N free space optical cross bar switch is easiest to understand conceptually.
FIG. 7 does not show any intervening lenses. The lenses may consist of anamorphic (cylindrical) lenses that perform imaging operations along one direction and collimation or focusing along the orthogonal axis. The two-dimensional switch consists of binary amplitude Spatial Light Modulators (SLM) 704, where each of the 4xc3x976 array of pixels may be transmitting or opaque. By controlling the transmittance of each of the pixels, any one of the 4 inputs 702 may be connected to any of the six outputs 706. This fully generalized switch may perform an arbitrary permutation, broadcast, or perform fan-in operation(s). This switch however, usually has low throughput (high loss). Each input channel is broadcast to all output channels. In a permutation operation, only one of the 6 pixels in a row in FIG. 7 above will be transmitting, hence the rest of the light is lost. This light loss is directly proportional to the size of the switch and hence for large size switches ( greater than 100), the losses may be unacceptable. A second undesirable feature is the slow switching speed of most 2-D SLM""s. This limits the reconfiguration time for the switch. The switch is relatively bulky and may require coupling from and to fibers. The generation and delivery of control signals may also pose difficulty. None-the-less, several such switches have been built and demonstrated by using ferroelectric liquid crystal SLM""s. Another realization of this overall architecture employs fibers, 1-N fiber splitters and combiners and semiconductor optical amplifiers as amplifying switches that control each cross point. The resulting system may be compact since fiber splitters now replace bulky lenses allowing easy interfacing to fibers. Switching the semiconductor optical amplifiers very fast, may overcome reconfiguration speed limitations. The gain provided by the optical amplifiers may overcome broadcast loss, improving overall throughput efficiency. However, semiconductor optical amplifiers are expensive, hard to couple to fibers and are polarization sensitive. The cross bar switch based on semiconductor optical amplifiers may therefore be limited to small sizes ( less than 16).
One way to avoid broadcast loses may be to use light deflection switches instead of light modulation switches. One example of such a light deflection switch is an acoustooptic device. In an acoustooptic device, a traveling phase grating is created within a transparent crystal in response to applied RF signal to a piezoelectric transducer attached to one end of the crystal. The frequency of the grating and hence the deflection angle is controlled by the frequency of the RF drive signal. FIG. 8 (prior art) shows a schematic diagram of such a cross bar optical switch 800.
The diffraction efficiency of the grating as well as the coupling efficiency at the output end into a fiber together determines the throughput efficiency of this switch. The deflection efficiencies may be quite high and indeed acoustooptic beam deflectors are routinely used in image scanning applications. The traveling nature of the gratings in the acoustooptic device may pose problems. A way around this utilizes multi-channel acoustooptic devices with a separate channel and transducer for each input channel. This may make the whole system bulky and power consuming. Therefore its use in communication and computer interconnection switching is not widely accepted.
A schematic diagram of another free space optical cross bar design employing the basic architecture shown in FIG. 6 is shown in FIG. 9 (prior art). The main switching element is a micro mirror fabricated out of thin membranes using Micro Electro Mechanical (MEMs) technology. Micro-lenses 940 collimate optical signals (possibly provided by single mode fibers) into small area beams. These beams travel parallel to the MEMs substrate. The MEMs substrate contains an array of micro-mirrors 920 and 930 that may move into the path of incoming optical beams. When activated, the movement could be out of the plane where the mirrors pop up and lie in the plane when deactivated. Alternatively, the mirrors could slide into a position to intercept the beam when activated and slide out when deactivated. The micro-mirrors may reflect the input optical beams to the appropriate output port. In the current illustration, micro-mirrors 920 are activated and micro-mirrors 930 are deactivated creating the permutation pattern corresponding to: input 1902 to output C 956, input 2904 to output B 954, input 3906 to output D 958, and input 4908 to output A 952. This design is compact and readily interfaced to fiber. Also, this design is passive optical, meaning that the optical signals may be correctly switched regardless of the bandwidth or format. Reconfiguration speeds could be moderate, into the 100 kHz range. Scalability of this design may be an issue. A primary limitation may be cross talk between channels due to diffractive spreading of small area optical beams. If the beam diameter is increased to reduce diffractive spreading, the channel density may decrease and the propagation distance may increase, thereby increasing the diffractive spreading. The alignment of the lenses and optical fibers and output coupling into single mode optical fibers may also be an issue.
What is needed is an optical switching system that is compact; robust; efficient; easily scalable to greater than 100 optical channels; accepts a multitude of formats, bandwidths and wavelengths of optical signals; integrates control signals easily; is easy to manufacture; has low cross-talk; and easily interfaces to optical fibers.
One advantage of the invention is that it allows for the construction of a compact, robust and efficient optical switch.
Another advantage of this invention is that it provides for an optical switch that may be easily scalable to greater than 100 optical channels.
Yet a further advantage of this invention is that it accepts a multitude of formats, bandwidths, and wavelengths of optical signals.
Yet a further advantage of this invention is that it integrates control signals easily, is easy to manufacture, has low cross-talk; and is easy interface to optical fibers.
To achieve the foregoing and other advantages, in accordance with all of the invention as embodied and broadly described herein, an optical switch comprising a plurality of optical waveguide switches. Each of the plurality of optical waveguide switches comprise an optical waveguide. The optical waveguide includes a substrate having a hollow channel having a beginning and an end running through the substrate, a plate residing atop the hollow channel, an input aperture at the beginning of the hollow channel, and an output aperture at the end of the hollow channel. The optical waveguide also includes a plurality of micro-mechanical deflectable cantilevered beam mirrors formed in the plate and oriented along the optical waveguide. The micro-mechanical deflectable cantilevered beam mirrors have a deflected position where the micro-mechanical deflectable cantilevered beam mirrors are operably deflected out of the plane of the plate into the hollow channel for intercepting an optical signal and deflecting that optical signal out of the plane of the hollow waveguide, and a non-deflected position where the micro-mechanical deflectable cantilevered beam mirrors remain in the plane of the plate. A plurality of mirror apertures are operably formed when each of the micro-mechanical deflectable cantilevered beam mirrors are in their deflected position. The optical switch also comprises at least two layers, each of the layers comprising at least one of the optical waveguide switches; an optical waveguide switch stack comprising at least two layers stacked so that at least one of the plurality of mirror apertures from each of the optical waveguide switches on adjacent the layers are aligned; at least one input optical interface, each of the input optical interfaces attached to one of the input apertures for receiving an optical signal from at least one external source; and at least one output optical interface, each of the output optical interfaces attached to one of the output apertures for the optical signal to pass from the optical switch to at least one external receiver.
In yet a further aspect of the invention, an optical switch wherein the plate is a membrane.
In yet a further aspect of the invention, a cross connect switch where one of the layers is a first layer and where at least one of one of the optical waveguide switches on the first layer is a multitude of the optical waveguide switches aligned in parallel. Another layers is a second layer where at least one of one of the optical waveguide switches on the second layer is a multitude of the optical waveguide switches aligned in parallel. The first layer and the second layer are adjacent layers. The first layer and the second layer are aligned so that the optical waveguide switches on the first layer are perpendicular to the optical waveguide switches on the second layer.
In yet a further aspect of the invention, an optical switch wherein the micro-mechanical deflectable cantilevered beam mirrors are operably deflected in a predetermined pattern that is synchronous with a time modulated optical signal. The time modulated optical signal may be a display signal.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.